For the same reason a University can cram over 100 students into a three hour course survey course at $200-1000/hr (maybe more) depending on the institution AND have it taught by a grad student making a pittance and that may or may not have passable (insert local country language here).

At the low end, that brings in $60,000 in revenue on a class they pay a grad student maybe $2000 to teach. Higher education economics baffle me completely.